Conventional vibratory apparatus for sifting mud and like material in the oil industry generally utilise screens of either hook strip or pretensioned design. Hook strip screens generally consist of single or multiple layers of mesh bonded together, which are tensioned after mounting in the basket of the vibratory screen apparatus. Two opposed ends of the screen are fitted with a turn back element to form a hook strip which is hooked around a tension rail, the latter being attached to the side wall of the basket, typically via a tension bolt, though other loading means to apply tensioning and securing forces may be employed. Tightening the tension bolt moves the tension rail outwardly, towards the walls of the basket, thus applying tension to the screen. The screen is normally stretched over a crowned deck, giving an arcuate profile to the screen, so that rigidity of the screen is retained during vibratory motion.
Hook strip screens may be pretensioned prior to mounting in the basket by attachment of the screen mesh element to an apertured support plate—typically by means of an adhesive. Where a plurality of mesh layers is used, these would normally be pretensioned. In some designs, layers of fused mesh may be corrugated prior to mounting to an apertured support plate and the hooks applied thereafter to the mesh-plate combination.
Hook strip screens have a number of disadvantages including the complex and time consuming mounting of the screen members in the basket, which results in significant downtime of the vibratory screen apparatus and requires the use of multiple parts. Attaining the correct screen tension for the material to be sieved also involves intricate fine tuning and the screens are easily damaged if too much force is applied when tightening the bolts or loading means to tension the screens. A further disadvantage is the relatively poor sealing between the screen and basket. The metal on metal seal often results in leakage with unscreened material passing through gaps between the screen and the basket, and mixing with already screened material below the mesh screen. Attempts to overcome the poor seal by placing rubber strips or gaskets at the metal/metal interfaces have proved unsatisfactory. These require intricate and time consuming fitting and frequently work loose during vibration and become lost or lodged in the vibratory machine, obstructing and damaging the machinery. In addition applying tension to the screen when tightening the tension bolt or loading means gives rise to additional stresses over and above those already suffered by the machine framework due to the vibratory motion, thereby possibly resulting in deterioration of the framework.
Pretensioned screens generally comprise one or more layers of mesh permanently bonded under tension onto a generally rigid steel and/or plastics material apertured plate support frame. The mesh screen may be flat, or crowned. The screen and frame is inserted into the basket as a unit, requiring no adjustment to the tension of the screen. The screen and frame unit is normally secured in the machine by clamping it from above or below by, but not restricted to, hydraulic pistons, inflatable clamping bags, bolts, or tapered elements.
Conventional pretensioned screen units with integral support frames have significant disadvantages being bulky, heavy and difficult to handle, transport and store; all very major considerations for use on off shore installations where all of these are at a very high premium. The design is complex and the frames expensive to construct. Typically plastic injection moulding is used which is an inflexible method of construction. The frames utilise large amounts of material all of which require disposal when the screen units are replaced, which is both inconvenient and expensive and has a negative impact on the environment.
Attempts to address some of these problems have been made (GB 2245191) by using a filter screen assembly comprising a main frame of moulded plastics material, subdivided by cross pieces which secure a stretched screen mesh. When a sub area of mesh is damaged it may be plugged with a module, snap fitted in place of the mesh. These screen assemblies, however, are very complex in structure with many of the disadvantages and associated expense of conventional pretensioned screens and, in addition, the inconvenience, storage and additional material requirements of the modules.
A further problem that arises with known screen systems is that due to the extra weight of the material being processed which is concentrated, in use of screening apparatus at the back (upstream) end thereof. The screen mesh in that area is subjected to particularly heavy wear resulting in the frequent need for replacement thereof. This problem has previously been addressed by using coarser heavy duty mesh in that area, but this inevitably results in reduced screening performance of the screening apparatus.